Variable output hydraulic system using fixed displacement pump and variable opening venturi pump



July 10, 1962 A. B. MAGNUS, JR 3,043,107

VARIABLEOUTPUTHYDRAULICSYSTEMUSINGFIXEDDISPLACEMENT PUMPANDVARIABLEOPENINGVENTURIPUMP Filed Jan. 5, 1960 2 Sheets-Sheet l INVENTOR. Qn/@V056 LMA/af, Je.

July 10, 1962 A B. MAGNUS, JR 3,043,107

VARIABLE OUTPUT HYDRULIC SYSTEM USING FIXED DISPLACEMENT PUMP AND VARIABLE OPENING VENTURI PUMP Filed Jan. 5, 1960 2 Sheets-Sheet 2 tre This invention relates to hydraulic systems capable of delivering variable iiow, or output, at a predetermined pressure range, by means of a fixed displacement pump with a variable area Venturi pump connected in series with the xed displacement pump. The Venturi pump returns that amount of fluid back to the input side of the fixed displacement pump which is in excess of the flow 3,043,107 Patented July 10, 1962 ble, lightweight and compact, and cheaper constant speed fixed displacement pumps, rather than the variable displacement pumps for delivering a variable rate of flow at a substantially constant pressure, or at least a constant pressure range. Y

An additional advantage of the disclosed system resides in the fact that that portion of the potential energy of the pressurized fluid which is not required by the hydraulic system load, is usefully employed to pressurize the inlet of the fixed displacement pump to suppress cavitation effects and to conserve energy. The reduction in total pump pressure rise results in a corresponding reduction in pump input power, thereby reducing the total energy requirements of the fluid pressurizing system While the required by a variable hydraulic load connected to the output side of the constant displacement pump and operated by the pressurized fluid. The invention also discloses a control system for controlling and maintaining within a predetermined range the hydraulic outlet pressure of the constant displacement pump. The control system also serves automatically to vary the variable area opening of the Venturi pump to match load demands.

The disclosed system is applicable to any hydraulic system utilizing a fixed displacement pump to operate into into a variable demand hydraulic load. The output demands of such devices may vary and the fixed displacement pumps for such systems are sized to take care of maximum load requirements. Accordingly, when such pumps are operated at substantiallyconstant speed, which is usually the case, reduced load demand requires recirculation of the excess pump output tiow around the load. The amount of the recirculated fluid around the hydraulic load is a function of the system demand and of the rated capacity of the pump.

Due to their mechanical simplicity, fixed displacement pumps are more reliable and less expensive than the equivalent variable displacement pumps and, therefore, fixed displacement pumps are preferred where possible.

In order to vary the volume of flow delivered by the fixed displacement pump, it is yalso possible to interpose a variable speed transmission between a constant speed prime mover and the fixed displacement pump, with the transmission output speed controlled by the flow demand of the system. Like variable displacement pumps, such systems also are less reliable and more costly than the iixed displacement pumps driven directly by the prime movers.

The disclosed system has the inherent simplicity and reliability of the fixed displacement-constant speed drive system. The disclosed system also provides a novel recirculating circuit and novel control means for this recirculating circuit, the circuit and the controls having the same high reliability and mechanical simplicity as those typical of the fixed displacement pump.

A further advantage of the disclosed system is the reduction in fluid heating and total energy input requirements. In systems utilizing a throttling relief valve, the throttling of pressurized fluid heats the fluid by converting pressure potential energy to thermal energy. In missile hydraulic power supplies, the principal disadvantage of such heating is the wasting of the power contained in the recirculated ow, thereby requiring additional weight and space for batteries or some other energy source used for driving the pump.

It is the purpose of the disclosed Venturi pumping system to-effect a substantial reduction in system energy input and associated oil heating during periods of reduced hydraulic system demands.

Additional advantages, typical of the disclosed fluid pressurizing method, is the utilization of the more reliacontrol system maintains the output pressure of the system within the limits of the desired pressure range.

It is, therefore, an object of this invention to provide a constant speed fixed displacement pump system for pressurizing `a working hydraulic fluid in which excess fluid delivered by the pump is recirculated continuously around the pump through a variable area Venturi pump connected in series with the input side of the fixed displacement pump, such recirculation being obtained with a minimum of heating of the working fiuid and substantial reduction in pump input power through the entire operating range of the system, except at full load when the Venturi pump nozzle is shut off.

It is an additional object of this invention to provide the hydraulic pressurizing system of the above type in which the degree of opening and closing of the variable area nozzle of the Venturi pump is controlled by a differential pressure derived from the high pressure'appearing on the output side of the fixed displacement pump and the low pressure appearing on the input side of the Venturi pump.

Still another object of this invention is to provide pressure controls for the system of the above type which maintains the high pressure of the system within a predetermined range throughout the operating range of the system, from zero external load to full load.

The novel features which are believed to be characteristic of this invention, both as to its organization and method of operation and design configurations, together with further objects and advantages thereof, will be better understood from the following description given in connection with accompanying sketches in which embodiments of the invention and associated design configurations are illustrated by way of several examples. It is to be understood, however, that the drawings are for the purpose of illustration only, and are not intended as a definition of the limits of the invention.

FIGURE l is a schematic system diagram of a hydraulic system incorporating the disclosed invention.

FIGURE 2 is a longitudinal section of a direct acting Venturi pump Iassembly incorporating la variable area pintle valve and controls for the pintle.

FIGURE 3 is a schematic diagram of a hydraulic system with the variable area pintle valve being positioned by an actuator, and controlled by a servo system.

FlGURE 4 is an enlarged view of the Venturi pump and its controls which are used in FIGURE 3.

The invention will be described in connection with a hydraulic supply system in which hydraulic oil is delivered at an essentially constant volume flow rate by means of a fixed displacement pump, the pump being operated at constant speed, and excess hydraulic fluid is recirculated back to the inlet of the hydraulic pump by means of a single stage variable oriiice opening Venturi pump.

Referring to FIGURE l, the reservoir 10l is filled with a suitable hydraulic uid, usually oil, 11. The oil is returned to reservoir itl through a return duct, or pipe 12 `which is provided with an optional hydraulic accumulator assembly 13 having a piston 15. Accumulator 13 acts in usual manner as a surge-arresting means and as a hydraulic fly-wheel. Reservoir supplies oil to the pressurizing system throughran yinput duct 16 connected to the low pressure chamber 17, FIGURE 2 of the Venturi pump 18 incorporates 2. duct 19 provided in housing 29 of the Venturi pump. Duct 19 connects to a branch duct 2 1 which is also connected to hydraulic passage 22 around the entire periphery of slide 64,.at the differential diameter -sections 68 and 67. The Venturi pump will be described more in detail upon the conclusion of the description of FIGURE 1. The low pressure chamber 17 is connected to the throat portion 26 of theV Venturi pump and the throat merges into a constant diameter mixing g tube 27 and then a diffuser 28 which discharges the partially pressurized oil into a duct 30. Duct 30 is connected to the input side of the fixed displacement pump 31 which discharges iiuid at system pressure to pressurized duct 114. Because the throat of the Venturi pump would choke the flow in the series circuit 1 6-18-39--31-14-80f- 12 at full load or substantially maximum flow, a bypass circuit 32-33-34 is provided which shunts the Venturi pump through ducts 32 and 34 and a bypass check valve 33 provided with a poppet 35, guide rod 36 and a compression spring 37. When pressure in the input duct 30 and shunting duct 34 become sufliciently low at high rate of ow of the iluid through the system, the pressure in duct'32 is suticiently. high (30-60 p.s.i.) to open valve 33. -Fluid from reservoir ltl then flows directly'to the input side 30-of pump 31 through ducts 16, 32, 34 and 30 and bypass valve 33. Venturi `pump pintle 38, FIGURE 2, is then fully closed, as illustrated in FIGURE 2, and the YVenturi pump is not in operation. Therefore, for practical purposes, the entire ilow takes place through the low resistance-to-ow shunt circuit.

An additional bypass circuit includes ducts 39 and 40 with a relief valve 41 which opens only.when the pressure of the iiuid in ducts 14 and 42 becomes higher than some rated maximum overpressure. Therefore, the second bypass circuit is a safety device which prevents destructive overpressures due to malfunctioning of the Venturi control. VDuct 42 is the duct which connects duct 14 to the /tion ofthe kinetic energy-of the mixed pumped and pumping streams in the form ofV increased pressure at the inlet of the fixed displacement pump 31 which is driven by a i suitable.. prime mover connected to the pump shaft 47. z

The only remaining duct in FIGURE 1 that needs mentioning -is a duct 48 which connects the low pressure side of the shaft seal'of pump 31 Vto duct 40, thus requiring this seal to work across the lowest diierential circuit pressure available.V Y

Referring now to FIGURE 2, and concluding the description of this figure, anY end plug 5GY closes off the hollow cylinder 51 provided in housing 20 of the Venturi pump. Plug 50 is provided with an O ringr52 and a Belleville spring 53 which presses against sleeves 23 andY 54 and thus holds them'xed with respect to housing 20,

with sleeve 54 Iabutting against a shoulder 56 provided Y `in housing 20. Sleeves 23 and 54 are two separate cylinders, `and they are mechanically loaded to each otherby Y means of a plurality of radial ribs 57, 58, etc., which Vare uniformly distributed and spaced from each other aroundrthe inner end of sleeve 23. 'Ihese ribs, therefore, perform Vtwo functions; They allow the uid to `reach the Yinner low pressure chamber 22v through duct 19, duct 21, the spacings between the ribs and then into chamber 22; the second function is that they abut against sleeve 54 and thus hold the two cylinders 23 and 54 in proper relationship with respect to each other. The two sleeves 54 and 23 are provided with O ring packings 60 and 61 to prevent the ow of pressurized fluid between bore 51 and sleeves 23 and 54. The nature of the high pressure chamber 62 is explained below. The two sleeves 54 and 23 contain slides 63 and 64 which are of slightly different diameter. These slides constitute a single precision lapped assembly. A relief portion 66 is provided in slide 44 and it is madesuciently deep so as to produce two exposed areas 67 and 68 on which the uid in chamber 22 exerts pressure in two opposite directions. Slide 34 and 64 and its respective sleeves 54 and 23 are lapped with respect to each other so as to prevent any leakage of high pressure fluid from the chambers 43 and 62 into low pressure chamber 2.2. Chamber 43 is joined to chamber 62 by means of a duct 70 and, therefore, equal fluid pressure per unit of area is exerted by the fluid on the face lof slide 63 and 64. As mentioned slide 64, however, has a smaller diameter and, therefore, the total pressure force developed on the face of slide 64 is smaller than the total force acting on slide 63. The inequality between the two pressure `forces is balanced by vmeans of a compression 'spring72 mounted between an adjustable spring cap 73 and the face of slide 64.

The force exerted by spring 72 may be adjusted by adjusting the kposition 'of cap 73 on plug 50. Pintle 38 is attached to and moves with slide assembly 63, 64; and therefore, its position with respect to orifice 45 is controlled by the unbalanced pressure forces.

The operation of the system disclosed in FIGURES 1 and 2V is as follows: Fluid ilows from reservoir 10 to duct 16, Venturi pump 18, `duct 30, xed displacement pump 31, duct 14, the hydraulic motor 80, duct 12 and back Vto, reservoir 10. Fixed displacement pump 31 is driven at constant, or substantially constant, speed by any suitable prime mover (electric motor; turbine; jet engineyetc., not shown in FIGURE 1) connected at shaft 47 of the pump.V During periods of peak system ilow, little orrn'o hydraulic fluid is recirculated through the Venturi pump 18 and, therefore, no iiuid flows in line 42 to the Venturi unit 18. 'As a result of the eiective restrictions imposed by the throat 26 of the Venturi pump a low pressurebypass circuit is aifordedby line 32, connecting through the bypass check valve 33 to line 34, and thence directly to the inlet of the hydraulic pump 31. Whenever the pressure in cavity 5 is connected to line 34 is less than the pressure in line 32, the bypass valve opens under the action of the differential pressure force, affording a low impedance bypass ow path around the Venturi pump at maximum ilow rate, i.e., at full load. During periods of reduced system demands, the excess of uid, delivered by pump 31 ows to the variable area Venturi pump 18 through line 42. YHigh pressure fluid from pump 31 is delivered by line 14 to accumulator 13 and to the hydraulic system motorsV and then back to reservoir 10. The function of lthe Venturi pump 18 is to convert 4the pressure potential energy in the high pressure bypass flow to kinetic energy at the throat section of the'Venturi; diffuser 28 thenV recovers a portion of the kinetic energy in the form of increased pressure at the inlet of the xed displacement hydraulic pump 31. The advantage 'of the Venturi pump is that it may operate over a variable flow and pressure range in lines 16, 14 and 42. Y

The Venturi pump control means, including (see FIG. 2) slide 63-64 spring 72, and pressure cavities 43,62 and 22, are so arranged that the Venturi pump maintains substantially constant pressure in duct 14, on the output side of the iixedy displacement pump-31. This is accomplished as follows: When the bypassV pressure in cavity 43 tends to increase because of the decrease in the external load connected to motor Sti', the slide 6.3-64 is moved to the rear (left, in FIGURE 2) against the spring 72 until a lforce equilibrium is achieved. The rearward movement of slide 63-64 opens oriice 45 wider than it has been before because of the rearward displacement of pintle 38, lincreasing the effective flow area 45, thus increasing the bypass flow through the Venturi pump yand tending to decrease the pump discharge pressure to a new steady-state value. In this process of establishing a new slide piston, the discharge pressure on the output side of the constant displacement pump 31, the low pressure appearing in ducts 16, 19, Z1, and chamber 22, and the spring parameters, are instrumental in determining the nal position of pintle 38.

In the light of the above description, it follows that the control system of the Venturi pump in this case is arranged to function between the output pressure in duct 14 and the reservoir 10 pressure, with the latter acting as a constant reference pressure. The output pressure in duct 14 is maintained essentially constant by varying the amount of fluid flowing through the Venturi pump as a function of the output pressure in duct 14, i.e., increasing pressure causing more bypass flow.

FIGURES 3 and 4 disclose an additional version of the system for maintaining constant the pressure of the uid on the output side of the fixed displacement pump. The main connections of the Venturi pump and of the ixed displacement pump are identical in FIGURES 1-2 and 3`4 and, therefore, they bear the same numbers. The main difference resides in the control system which is illustrated on an enlarged scale in FIGURE 4. It employs a double acting actuator controlled by a four-way servo valve. The four-way valve is operated by a pressure sensor and the net balance of pressure forces across it to produce the identical action to the system described above.

Repeating the main connections, reservoir 1i! is connected to duct 16 which in turn is connected to the low pressure chamber 17 of the Venturi pump 18. The diffuser 28 of the Venturi pump 1S is connected to an outgoing duct 30 which is connected to the fixed displacement pump 31. The output of pump 31 is connected to duct 14 which in turn is connected to the hydraulic load 89 and then to the return duct 12. An optional accumulator 13 is connected in parallel with duct 14 for absorbing any hydraulic surges that may be encountered by the system. Accordingly, the Venturi pump I18, as in FIGURES 1 and 2, is connected in series with the xed displacement pump 31 and is used in the same manner and for the same purpose as the Venturi pump 18 in FIGURES l and 2. The shunting, or the recirculating, circuit of this system is identical to that in FIGURES 1 and 2: It includes duct 42 connected in shunt with duct 14, the downstream portion of this duct being connected to the high pressure chamber 43 of the Venturi pump. This high pressure chamber on the downstream side terminates in the discharge orice 45 of the Venturi pump, the opening of which is controlled, as in the case of FIGURES l and 2, by the differential pressure derived from the high pressure p2 appearing on the output side of the fixed displacement pump and the low pressure p1 `appearing on the input side of the Venturi pump 18, which is essentially equal to the pressure of the fluid in reservoir 1 0.

The control system of the Venturi pump differs from that illustrated in FIGURES l and 2, although its function is identical to that of the control system disclosed in the earlier ligures; namely, it varies the opening of orice 45 by varying the position of pintle 38 and in this manner it varies the bypass flow, through duct 42.

The connections of the control system are illustrated in FEGURE 4. The Venturi pump 18 includes an outer cylinder 32S, an intermediate cylinder 310, and an inner cylinder 312. The outer cylinder 328 constitutes the envelope of the low pressure chamber 17, the intermediate cylinder 310 provides an envelope for the high pressure chamber 43, and the inner cylinder acts as a cylinder for a double acting actuator for pintlev38, connected to a piston 3114. 'Ihe left chamber 316 of cylinder 312 is connected via a duct 306 and the right chamber 315 is connected via a duct 365. These two ducts are connected to a four-way servo valve 340, which includes a sliding spool with a push-rod 325, with three lands 326, 327 and 32S, mounted in a cylinder 341 and actuated by the double acting servo motor 300. The pressure servo motor 3410, senses the pressures in ducts 3011 and 302, and responds to the difference in the two pressures, includes a piston 321, a cylinder 329, compression springs 323 and 324, a spring plate 342, a micrometer screw 344, and a knurled knob 346 at the end of the screw. Cylinder 341 has low pressure ducts 329, 33t) and 331 connected respectively to chambers 347, 348 and 349, and a high pressure duct 333 connected to a central chamber 350. Chambers 347, 348 and `349 equalize the internal pressures, produced by the low pressure on the spool of the valve, these three chambers being connected to each other through duct 304. Duct Sil/4 is connected to the input duct 15, and, therefore, this circuit and chambers 347, 343 and 349, are at the reservoir 10 pressure. The high pressure appearing in chamber 356i also has no eect on the spool. The spool is moved to the left, as Viewed in FIGURE 4, when the pressure in chamber 360 is decreased while the pressure in chamber 361 remains constant. The pressure in chamber 36d decreases when the high pressure appearing in duct 14 on the output side of the constant displacement pump 31 momentarily decreases due to the increase in the flow of the fluid to motor Si), which, in turn, lis caused by the increase in the external load. The spool and its rod 325 are moved to the left, duct 306 lbecomes connected to duct 393, and piston 314 shuts off orice `45, thus restoring the pressure in duct *14 to its original value. At the same time, duct 305 becomes connected to duct 331, which connects chamber 315 to the low pressure of Ireservoir 10. The Venturi pump `becomes disconnected altogether at high external loads and it is wide open at no load, with the degree of opening of orifice 45 being a function of p2-p1-='Ap, where Ap is `a function of the change in p2, or Ap2, since it may be assumed that p1 remains substantially constant. By varying the opening yof orifice y'45, the Venturi pump maintains substantially constant p2 and reduces the transitional tluctuations in p2 to a 'Ihe pressure transients are also `controlled by accumulator 13, which acts as a pressure fly whee It has been already stated in the introductory part of the specication that the Venturi pump system alsoreduces heating of the uid because of the elimination of the conventional fluid throttling, `and the reduction of work performed by pump 31 at -any other load other than the full load `condition due to the conveyance of the excess high pressure fluid back to duct 30 where -it appears as an increase in pump inlet pressure.

What is claimed `as new, is:

l. A uid supply system for receiving uid at pressure p1 and delivering said fluid at a substantially constant pressure p2 to a variable external load as said load varies between 0% load and 100% load, said system including constant displacement and a substantially constant rate of ow pump delivering said iluid at a substantially constant rate, a Venturi pump connected to the input side and in series with said constant displacement pump, a variable orifice in said Venturi pump, a pintle for varying the opening of said orifice, said Venturi pump receiving the uid at said pressure p1, and discharging it into said xed displacement pump at a variable pressure p3, and control means for operating `said pintle, including a recirculation duct connected to said Venturi pump to vary the yamount of the recirculation of the fluid diverted to the recirculation duct in direct proportion to the amount of uid delivered by said ixed displacement pump in excess of that required by said external load, said control means differential control means includes a four-way servo valve,

a second spool Valve having a slide connected to and positioned by said servo valve, -a Variable opening nozzle in said Venturi pump, and a double-acting actuator controlling the degree lof opening of said nozzle, said actuator being hydraulically connected to said spool valve, said diiierential control means being :arranged to maintain said Y p2 substantially constant.

pression spring for equalizing the inequality of opposingV forces exerted by saidrtluid in said piston when p2 and p1 have a predetermined value.

, 4; The `system as deiined in claim 3, in which said Venturi pump includes an orifice and a pintle for closing and openi-ngisaid oriiice, said pintle being connected to one of said pistons for actuating said pintle so as to close said orifice completely when the external load is the 100% load and to open said orifice completely when said load is the load.

5. A hydraulic system for operating a variable load by means of a compressed fluid, said load having variable fluid requirements, said system comprising a series circuit including a source of fluid, a Venturi pump, a constant displacement pump having means for operating said pump at substantially constant speed, whereby said pump delivers a substantially constant rate of iiow of a working iiuid, and said load receiving said iiuid from said constant displacement pump and returning the spent fluid to said source; low and high' pressure chambers in Said Venturi pump, said low pressure chamber being hydraulically directly connected to said source on the upstream side and Vdirectly connected to Y said constant displacement pump on the downstream side; and said high pressure chamber being hydraulically directly connected on the upstreamrside of the discharge side oi' said constant displacement pump, an orifice interconnecting said chamber-s,a pintle for varying the degree of opening of said orice, and control meansV for closing said orifice only at full load Iand for varying the position of said pintle cuit including: a source of fluid under low, but above atmospheric, pressure, an input duct, said input duct terminating in a lowV pressure chamber of a Venturi pump, said low pressure chamber opening and merging on one side into -a throat, a mixing chamber and then a dituser of said Venturi pump, a constant displacement pump l connected to said ldiiuser; a high pressure, obstructionfree output duct connected directly to the high pressure sideof said constant displacement pump, a hydraulic load hydraulically connected directly to said constant displacement pump through said high pressure duct, and `a return duct between said load and said source completing said series circuit; .said Venturi pump also including a high pressure chamber adjacent to and separated by a wall from said low'pressure chamber, an oriiice opening in said wall, a pintle for closing and opening said orice, a piston-cylinder combination, said piston being connected to `and controlling the position of said Vpintle for varying the degree of opening of said oriiice, and hydraulic means for making said piston responsive to la diiierence in pressure of a working iluid a't said source and said output duct, said means making said orifice wide open at minimum hydraulic load and closed at full load, whereby said Venturi pump remains in said series circuit throughout the operating range of said system except the full load.

7. A uid pressurizing system as defined in claim 6 which also includes a bypass duct and a check valve in said bypass duct, said bypass duct interconnecting said source and the input side of said constant displacement pump, said check valve having spring means to keep said valve closed except at full load of said system.

8. A fluid pressurizing system vas dened in claim 6 in which said Venturi pump and said hydraulic means keep the pressure of said uid constant on the output side of said constant displacement pump.

9. A iiuid pressurizing system comprising a source of fluid at an above atmospheric pressure p1, a constant speed and a constant displacement pump operated at said substantially constant speed and substantially constant rate of flow of a Working fluid, said constant displacement pump pressurizing said uid to a pressure p2, af'variable external load connected to the output side of said constant displacement pump, a Venturi pump having low and high pressure chambers, an orice between said chambers, a-

pintle for opening and closing said orice, and a control system for actuating said pintle; said 'low pressure chamber with respect'to said oriiice during any part load so as to vary the degree of opening of said oriiice to maintain constant the pressure produced by said constant displacement pump by continuously returning all of the excess iluid appearing on the downstream side of said constant displacement pump backA to its input side directly through said high and low pressure chambers and said variable opening orice Vwith only a limited rise in temperature in said fluid because of the limited throttling of said fluid Y Vclosing position lwhen said'Ap and p2 tend to be low and into an orilice-opening position when said Ap and pg tend to be high.

6. A fluid pressurizing system comprising a series circonnected to the vhigh pressure side of said constant displacement pump, said control system having means for making the position of said pintle responsive to the difference between p2 and p1 so as to keep said oriiice wide open when said external load is minimum and closed when said load is maximum and simultaneously keeping said p2 substantially constant, whereby saidVenturi pump remains in series with said constant displacement pump throughout the operating range of said system except at the maximum load.

l0. The fluid pressurizing system as dened in claim 9 in whichrsaid control'system includes a double acting servomotor actuated by -a sensor pressure Ap, Where Ap is equal to p2-p1, a four-Way servo valve mechanically connected to and actuated by said servomotor, a cylinder and a piston within said cylinder, said piston/being connected to said pintle, and high and low pressure ducts connected to said cylinder through said four-Way servovalve ,for actuating said pistoncand said pintle to maintain saidV p2 substantially constant and to return all excess uid Working fluid, a low pressure chamber of a Venturi pump hydraulically directly connected to said source, a diffuser receiving fluid from said low pressure chamber, a substantially constant rate of flow and a constant displacement pump hydraulically directly connected to said diuser, a hydraulic motor connected between the output side of said constant displacement pump and said source for supplying power to a variable external load, and a circuit for returning all excess rate of oW of said constant rate of oW to the input side of said constant displacement pump throughout the operating range of said system except when said external load is maximum, said circuit including a high pressure chamber of said Venturi pump directly connected to the output side of said constant displacement pump, said high pressure chamber being adjacent to said 10W pressure chamber, an orice hydraulically connecting said high pressure chamber to said low pressure chamber, a pintle for closing and opening said orifice, and control means for actuating said pintle so as to close said orice at full external load and have it Wide open at minimum external load -for returning said excess rate of dow to the input yside of said constant displacement pump, While maintaining substantially constant the pressure of the fluid on the output side of said constant displacement pump,

said control means vfor actuating said pintle including first and second abutting cylinders of diierent diameters, first and second mechanically interconnected pistons mounted Within `said first and second cylinders, respectively, each piston having an external piston head, means for exposing both of said heads to a pressure p2 of the uid on the output side of sm'd constant displacement pump, a diierential area at the junction of said pistons, means for exposing said differential area to -a pressure p1 of said uid at said source, and a connection between one of said pistons and said pintle for operating said pintle in response to the difference between p2 and p1.

References Cited in the file of this patent UNITED STATES PATENTS 1,350,095 Eddison Aug. 17, 1920 2,170,890 Allen Aug. 29, 1939 2,246,678 Harris .Tune 24, 1941 2,251,664 Davis Aug. 5, 1941 2,457,388 Lung Dec. 28, 1948 2,802,337 Bunch Aug. 13, 1957 2,829,599 Jones Apr. 8, 1958 

